1. Field of the Invention
The present invention relates to a method and a motor driving circuit for adjusting a rotational speed of a motor, and more particularly, to a method and a motor driving circuit for adjusting a rotational speed of a motor when an input voltage of the motor enters into a predetermined voltage range.
2. Description of the Prior Art
A motor is an electronic device for transferring electrical energy into dynamic energy, such as a DC motor, an AC motor or a stepper motor, etc. The DC motor is frequently utilized in non-sophisticated control devices, such as a fan. Generally, the DC motor rotates based on a current passing through coils of a stator of the DC motor to generate different amounts or polarized directions of magnetic force to attract or repel a permanent magnet on a rotor of the DC motor to make the motor rotate.
For noise problems in a dissipating heat fan, U.S. Pat. No. 8,063,601, owned by the applicant, discloses a motor driving circuit designed to alter an output voltage for adjusting a rotational speed of the motor, as shown in FIG. 1. The motor 18 can be applied to a dissipating heat fan or other devices, which have one end coupled to a variable voltage source VIN and another end controlled via a motor driving circuit 1. The motor driving circuit 1 includes a motor-driving unit 10, a circuit control unit 12 and a determining unit 14. The circuit control unit 12 further includes a voltage-dropping device 120, a first switching device 122 and a second switching device 124.
The motor driving circuit 1 switches between the first switching device 122 and the second switching device 124 via the circuit control unit 12 to comply with unit-voltage differences provided by the motor-driving unit 10 and the voltage-dropping device 120, which forms different conduction paths to adaptively lower or elevate a terminal voltage of the motor 18 and correspondingly slow down or speed up the rotational speed of the motor 18. Under such circumstances, large amounts of elements/units are used to adjust the terminal voltage and results in a longer responsive period. Besides, the motor 18 must be switched between these two conduction paths. The circuit control unit 12 is utilized to accurately control the switching process or other error-proof circuits must be installed to avoid unnecessary errors during the switching process. Therefore, it has become an important issue to provide another motor driving circuit to adjust the driving voltage source directly passing through the motor to adaptively adjust the rotational speed on the motor.